Hydrometallurgical processes involve the use of aqueous chemistry to recover metals from ores, concentrates, tailings, slags or other materials, and can be typically divided into three general categories: leaching, concentration/purification, and metal recovery.
Leaching involves the dissolution of desired metal components into an aqueous phase, by contacting an aqueous solution that contains leaching agents, called leach solution or lixiviant, with the metal bearing material. The lixiviant may be acidic or basic in nature. Common leaching agents are sulphuric acid, hydrochloric acid, nitric acid, phosphoric acid, carbonic acid, citric acid, acetic acid, formic acid, ammonia, cyanide, urea, thiourea, thiosulphate, among others, together with salts such as sulphates, chlorides, nitrates, phosphates, carbonates, ammonium salts, acetates, peroxides, cyanides, formates, citrates, bromides, among others, including also oxidising and reducing agents like oxygen, hydrogen peroxide, calcium peroxide, sulphur dioxide, ferric nitrate, magnesium oxide, manganese dioxide, elemental iron, scrap metals, air, and others, together with catalysts and other additives. Some traditional leaching techniques are in-situ leaching, heap leaching, dump leaching, agitation leaching, vat leaching, and pressure leaching. In-situ leaching involves the introduction of the leach solution directly into the ore deposit, after opening and evaluating appropriate pathways for its penetration. Heap leaching is performed on crushed ore that is piled on a heap, typically after agglomeration and curing of the ore, allowing the lixiviant to percolate through the heap. In dump leaching a coarser ore, typically a run-of-mine ore without crushing, is loaded on a dump of increased height, allowing the lixiviant to percolate through the dump. Agitation leaching, also known as tank leaching or slurry leaching, involves material that is ground sufficiently fine so as to form a slurry or pulp, i.e., a fluid mixture of a pulverised solid with a liquid that can flow under gravity or when pumped by centrifugal pumps, being the tanks typically equipped with mechanical agitators or gas introduction equipment to achieve leaching by maintaining the solids in suspension in the slurry. In vat leaching the lixiviant percolates through a typically coarser material, loaded in a vat under flooded conditions. Agitation leaching is typically continuous while vat leaching is usually batch operated. Another process for leaching slurries is pressure leaching, which involves closed autoclaves or pressure vessels, whereby leaching is carried out at higher pressures and temperatures, e.g., the Sherritt Gordon ammonia pressure leaching process.
After leaching, the resulting pregnant solution or pulp with dissolved metals in most cases is subject to concentration and purification processes to increase the metal content and to remove undesired impurities. Such concentration/purification may include solvent extraction (SX), precipitation, sorption, among others. In solvent extraction the dissolved metals are extracted from the pregnant leach solution to an organic solvent, from where they are then stripped into an aqueous electrolyte solution. Impurities and contaminants sometimes are also removed in a similar way. Precipitation involves generating a solid precipitate from the pregnant leach solution either by cementation, whereby ions are reduced to zero valence with a reducing agent, or by crystallisation, whereby the solubility conditions of dissolved metals or contaminants are changed, e.g., by reagent addition, temperature change or evaporation. In sorption the dissolved metals or the impurities are extracted from the pregnant leach solution or pulp into a sorbent, from which they are then desorbed (or eluted) with an eluent to an eluate. Such a sorbent is usually an insoluble solid material to which another type of substance becomes attached by means of absorption, adsorption, or ion exchange (IX). Absorption refers to the incorporation of a substance in one state into another of a different state, e.g., liquids being absorbed by a solid or gases being absorbed by a liquid. Adsorption denotes the physical adherence or bonding of ions and molecules onto the surface of another phase, e.g., reagents adsorbed to a solid catalyst surface. Ion exchange involves a reversible interchange of ions usually between an insoluble solid material, called ion exchanger, and a solution phase. Ion exchangers can be unselective or have binding preferences for certain ions or classes of ions, depending on their chemical structure. Common commercial sorbents that have been used in large-scale processes include activated carbon, zeolites, clays, and ion exchange resins (also known as ion exchange polymers).
Concentration/purification is then usually followed by a metal recovery process, which may involve electrowinning, precipitation (cementation/crystallisation), among others, sometimes combined with smelting and electrorefining, to produce the final metal product, either in its metallic state or as a chemical compound. Typically, electrowinning and electrorefining respectively involve the recovery and purification of metals using electrodeposition of metals on the cathode, and either an oxidation reaction or a metal dissolution on the anode.
For example, hydrometallurgical copper production usually involves the traditional techniques of agglomeration, curing, and heap leaching in acid media, typically sulphuric acid, followed by solvent extraction and electrowinning, producing copper cathodes of great purity as final product.
The main goal of a hydrometallurgical process, and of extractive metallurgy in general, is to achieve high metal recovery at low capital and operational costs. One of the major costs typically involved in a hydrometallurgical process is the consumption of leaching agents per ton of processed ore and per kilogram of final metal product. Leaching is not necessarily a very selective process and often part of the leaching agents is consumed by certain reactive compounds and impurities present in the ore besides the target metals, particularly in the case of acidic leaching and low grade ores. If the consumption of leaching agents is too high, then the overall process may become economically unattractive. This fact increasingly becomes more important with the current trend of diminishing ore grades. The dissolution of other impurities does not only affect economical parameters such as consumption of leaching agents, but may also present other technical and environmental issues required to be solved in the further process steps, affecting, for example, the quality of the final metal product.
In the hydrometallurgical copper production, for example, impurities like iron, manganese, magnesium, chlorine, arsenic, among others, may be transferred due to chemical and physical entrainments through solvent extraction to electrowinning, affecting the quality of copper cathodes.
Some leaching agents, such as ammonia, are considered as more selective towards target metals such as copper, nickel, cobalt and zinc, among others, and less selective towards impurities such as iron, aluminium, magnesium. However, the ammonia type leaching agents are rather more expensive and volatile, particularly at higher concentrations. For the economic viability of the overall process it becomes therefore necessary to minimise their consumption, particularly by avoiding high evaporation rates and by reducing the loss of unconsumed leaching agents to tailings and entrainments, e.g., by recycling them back to leaching.
An example of a leaching agent being recycled back to leaching is given in the U.S. Pat. No. 4,165,264 “Ammonia leaching” to Satchell, which proposes an improved process for obtaining copper from copper sulphide by leaching with an ammonium carbonate solution, oxygen and recycled gaseous ammonia and carbon dioxide. The proposed process is rather complex and requires the addition of oxygen to oxidise copper sulphide during leaching, the presence of several filtering steps, generating heat to form gaseous ammonia and carbon dioxide, the addition of a strongly alkaline material like gypsum in several parts of the process, and the elimination of ammonia before the electrolytic recovery of copper in an acid medium.
Techniques for agglomeration, curing and heap leaching of copper ores with sulphuric acid are disclosed in the Canadian patent No 1156049 “Copper leaching process” to Domic, which provides a process for recovering copper from copper ore by crushing the ore, wetting the crushed ore with separate additions of water and concentrated sulphuric acid, agglomerating the wetted ore to form lumps, aging the agglomerated ore for at least about three hours to indurate the agglomerate lumps and solubilise copper, percolating a leach liquid through a layer of aged ore, withdrawing a pregnant leach liquid from the bottom of the layer, and recovering copper from such pregnant leach liquid by solvent extraction or liquid ion exchange followed by cementation or electrowinning, or by direct electrowinning or direct cementation from the pregnant leach liquid. The patent discloses a process only for leaching copper ores with sulphuric acid, followed by traditional means of metal recovery.
More recently, some patents have been filed on curing ore with ammonia. The U.S. Pat. No. 8,388,729 “Method for ammoniacal leaching” to Welham, Johnston & Sutcliffe provides a method for leaching one or more target metals from an ore by curing the ore with an aqueous solution of a curing agent, leaching the cured ore at atmospheric pressure through the application of an ammonium carbonate solution containing free ammonia, and passing the resulting pregnant leach solution to a means for metal recovery. The U.S. Pat. No. 8,486,355 “Method for leaching cobalt from oxidised cobalt ores” to Sutcliffe, Johnston & Welham proposes a method for leaching cobalt from a non-lateritic oxidised cobalt ore by curing the ore with an aqueous solution of iron (II) salts, sulphite salts, sulphur dioxide, or combinations thereof, leaching the cured ore through the application of an ammonium carbonate solution containing free ammonia, and then passing the resulting pregnant leach solution to a means for cobalt recovery. Both patents of these inventors disclose methods only for ammoniacal leaching of one or more target metals, whereby the resulting pregnant leach solution is afterwards passed to some traditional means of metal recovery like solvent extraction, ion exchange, precipitation or cementation.
The patents above consider different leaching methods and are not related specifically to sorption methods with (solid) sorbents. Particularly, they consider neither the option of having a sorbent present during the leaching stage so as to perform simultaneous sorption leaching nor the use of a sorbent later on in the process to recycle leaching agents or residual target metals back to leaching.
The last decades have seen many advances in sorption technologies for metal concentration and purification in extractive metallurgy, allowing the extraction of metals and impurities not only from solutions but also from pulps (slurries), with no need of costly solid-liquid separation for the latter. In the case of extracting from a solution the process is called in general sorbent-in-solution (SIS), whereas in the case of extracting from a pulp it is called respectively sorbent-in-pulp (SIP) or sorbent-in-leach (SIL), depending on whether the pulp is contacted with the sorbent after the addition of the leaching agents or together with them. Usually, the name of these processes makes reference to the specific sorbent involved, typically an ion exchange resin or activated carbon. In the case of an ion exchange resin the processes are respectively called resin-in-solution (RIS), resin-in-pulp (RIP), and resin-in-leach (RIL), whereas in the case of activated carbon the processes are respectively known as carbon-in-solution (CIS), carbon-in-pulp (CIP), and carbon-in-leach (CIL). Sorbent-in-solution (SIS) can be applied to the pregnant leach solution after the leaching step or to some other solution from which certain dissolved species are to be extracted or removed, and is often implemented by a series of columns containing the sorbent through which the solution flows in an upwards direction, in which case the process is called sorbent-in-column (SIC), and specifically resin-in-column (MC) or carbon-in-column (CIC) when the sorbent is respectively an ion exchange resin or activated carbon. In sorbent-in-pulp (SIP) the sorption by the sorbent may start before pulp leaching is finished, whereas in sorbent-in-leach (SIL) the sorption by the sorbent is performed simultaneously with pulp leaching. Both processes, SIP and SIL, are typically performed in a series of agitated tanks (or vessels) where a coarse-sized granular sorbent and a finely ground ore slurry are contacted in a staged counter-current manner, separating after each stage the sorbent from the slurry by screening. All three processes (SIS, SIP and SIL) require afterwards an elution or desorption process to extract the target metals or species from the loaded sorbent to an aqueous solution (typically in one or more columns), which is then treated by further traditional separation or recovery processes like electrowinning, precipitation (cementation/crystallisation), among others.
Examples of RIS and RIP processes are disclosed in the US patent application N° 2011/0030508 “Process for metal separation using resin-in-pulp or resin-in-solution processes” from Dreisinger, MacDonald & Shaw, in the U.S. Pat. No. 6,350,420 “Resin-in-pulp method for recovery of nickel and cobalt” to Duyvesteyn, Neudorf & Weenink, and in the U.S. Pat. No. 6,344,068 “Process for recovering gold from thiosulfate leach solutions and slurries with ion exchange resin” to Fleming, Wells & Thomas. These patents disclose processes for treating solutions or slurries containing dissolved metals by loading the metals onto an ion exchange resin, having in common that leaching is performed preferably before contacting the solution or slurry with the ion exchange resin and not simultaneously.
Improvements on CIP and CIL processes are disclosed in the U.S. Pat. No. 4,816,234 “Utilization of oxygen in leaching and/or recovery procedures employing carbon” to Brison, Elmore & Mitchell, and in the U.S. Pat. No. 5,288,302 “Method and apparatus for extraction of metal values from metal bearing ores” to Hallinan, whereby gaseous or liquid agents, e.g., oxygen gas, are added during or before the CIP or CIL process. The U.S. Pat. No. 4,778,519 “Recovery of precious metals from a thiourea leach” to Pesic discloses a method for desorbing precious metals, such as gold and silver, from activated carbon loaded from thiourea leach solutions by means of a CIL or CIP process. These patents relate to processes whereby metals dissolved in a leaching pulp are loaded onto activated carbon, followed by an elution step.
The U.S. Pat. No. 7,901,484 “Resin-in-leach process to recover nickel and/or cobalt in ore leaching pulps” to Mendes provides a RIL process for directly recovering nickel, cobalt, or both, whereby pulp leaching with the addition of an acid or base dissolves the metals of interest, adsorbing simultaneously the metals rendered soluble onto an ionic exchange resin. Following elution of the charged resin, purification of nickel and cobalt present in the eluate can be recovered by conventional methods, such as precipitation, solvent extraction and membranes. The patent discloses a RIL process for nickel or cobalt recovery wherein the leaching agents (sulphuric, hydrochloric or nitric acid, or ammonia) are added simultaneously with the resin to a pulp, preferably under atmospheric conditions and in stirred vats.
The U.S. Pat. No. 4,723,998 “Recovery of gold from carbonaceous ores by simultaneous chlorine leach and ion exchange resin adsorption process” to O'Neil provides a gold recovery process in which the gold content of ores is extracted by a simultaneous chlorine leach and ion exchange resin adsorption procedure. The patent discloses a RIL process for gold recovery from a ground refractory carbonaceous ore that is slurried with water, wherein mixing tanks or chlorination vessels are used to agitate the mixture of slurry, resin and chlorine providing compounds, and wherein the resins flow preferably counter-current to the ore flow.
As can be appreciated in the previous patents, metal extraction by sorption is performed either from a solution or from a pulp (slurry), after or during leaching. In the case of sorption from a pulp, usually some sort of agitation leaching is involved. In particular, in the prior art no reference could be found that relates to the claimed novelty of the present invention, namely to a simultaneous sorption leaching in the state of wet solids. Likewise, no mention was found to a process for metal extraction using a sorbent to scavenge or recycle leaching agents back to leaching, to diminish the overall leaching agent consumption of the process, as disclosed in the present invention. One objective of the present invention is to overcome drawbacks associated with the prior art, or to at least provide a useful alternative thereto.